The principle of edge-lighting is well known, and is used in signage, advertising displays, and flat-panel backlights such as those used to illuminate LCD screens. In the general application, light is introduced into the edge of an optically flat sheet of clear material and rebounded by total internal reflection. The major faces of the clear sheet are most typically in parallel planes. Light introduced in this manner will exit the sheet only upon reaching another edge, unless some other optical discontinuity is encountered.
The faces of a light guide panel may also be slightly skewed to yield a wedge shape, in which case some portion of light may progressively exit one or both major faces. Wedge shapes are more common to backlighting applications, although tapered panels are also know in the art of general illumination, advertising, and signage. Various systems have been devised to extract light from a edge-lit panel in a predetermined fashion. In edge-lit signage, the goal is often to illuminate a specific diffusive message or emblem. In the backlighting of electronic displays, the goal is most typically the even distribution of illumination. In both cases, electric lamps have been used in the past as the source of illumination. However, in many applications, solid-state sources such as light-emitting diodes have begun to supplant the use of filamented lamps or fluorescent tubes.